1. Field of the Invention
The present invention relates to a recording sheet. More particularly, the present invention relates to a recording sheet which forms a color image when contacted with an acid.
2. Description of the Prior Art
A recording sheet utilizing the color forming reaction between an electron donating substantially colorless organic compound (hereinafter referred to as a "color former") and an electron attracting solid acid (hereinafter referred to as a "developer") has been known for a long time. Examples of such a recording sheet are, for example, the pressure sensitive copying paper as described in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457 2,972,547, etc., the heat sensitive copying paper as described in Japanese Patent Publication No. 4160/1968, U.S. Pat. No. 2,939,009, etc., and the recording member as described in German Patent Laid Open (OLS) No. 1,939,624.
These recording sheets will be explained by reference to a pressure sensitive copying paper. A pressure sensitive copying paper comprises an upper sheet produced by providing a microcapsule layer containing a color former on a support, an intermediate sheet produced by providing a microcapsule layer as described above on one side of a support and a developer layer on the other side of the support, and a lower sheet produced by providing a developer layer on a support. When the above sheets are arranged in such a manner that the microcapsule layer and the developer layer are contacted with each other and a pressure applied, the capsules are ruptured at the areas where the pressure is applied, and thus color is developed. In general, in these recording sheets, the developer layer is provided on the whole surface of the support and thus a desensitizing agent is print-coated on the areas of the sheet where recording is not necessary (as disclosed in U.S. Pat. No. 2,777,780).
However, these pressure sensitive copying papers have disadvantages in that undesired color images are formed. For example, when the microcapsules of the above described intermediate sheet are pressed, the color former solution contained in the capsules permeates through the support and diffuses into the developer layer provided on the opposite side of the support, and thus color-development results (permeation color-development). The desensitizing agent to be print-coated on the developer layer or color developer contains an organic solvent for dissolving a binder. Thus, when the developer layer at the areas where the desensitizer is not coated and the developer layer at the areas where the desensitizing agent is spot-printed are contacted with the microcapsule layer, the capsules are ruptured by the solvent and the color former is extracted from the capsules even though no pressure is applied, and thus color-development results (color fogging). Furthermore, although it is desired that the developer layer at the areas where the desensitizing agent is printed is not color-developed with the application of pressure and the like, the sizing agent and filler contained in the support react, as a matter of fact, with the color former, and thus color-development often occurs at these areas (color-development by the support).
Since the above described undesired color-development reduces the contrast, sharpness, resolving power, and the like of the copied image, the value of the pressure sensitive copying paper is reduced.